Photoconductive pickup tube having opaque gold pattern encapsulated in tin oxide layer



July 7, 1970 J. R. LEAMAN PHOTOCONDUCTIVE PICKUP TUBE HAVING OPAQUE GOLD PATTERN ENCAPSULATED IN TIN OXIDE LAYER Filed Sept. 26, 1967 if Z6 da INVENTOR JOEL R.. LEAMAN BY o/paaea/ A TTORNE Y United States Patent O U.S. Cl. 313-65 2 Claims ABSTRACT OF THE DISCLOSURE A photoconductive target with one or more reference areas opaque to light, for service as dark current reference means and reticles. The reference areas are defined by layers of gold having an opaque thickness interposed between two layers of a tin oxide signal electrode.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to pickup tubes of the photoconductive type, and particularly to a target for such a tube.

Description of the prior art A photoconductive pickup tube such as a vidicon normally includes an evacuated elongated envelope having an electron gun in one end thereof adapted to produce a beam of electrons. In the other portion of the envelope is disposed a target comprising an insulating substrate, a transparent conducting signal plate on the substrate and a layer of photoconductive material on the signal plate. The beam is caused to scan the target by suitable means such as a system of coordinate electromagnetic dellecting coils positioned outside of the tube envelope.

When an image is focused upon the photoconductive layer, the lighter portions of the image render elemental areas of the photoconductive layer conductive. However, it is found that the photoconductive layer is characterized by some conductivity even in the absence of light thereon. This conductivity is relatively 10W and is known as the dark current level of the photoconductor.

Although the dark current level of the photoconductor is relatively low, it must be taken into account for proper operation of a vidicon. Thus, Where the dark current level at a given temperature of operation is known, the voltages on the `signal electrode and on the cathode are adjusted to produce a signal level that is at all times above the dark current level. Such adjustment is feasible due to the fact that a change in the voltage difference between the signal plate and the cathode has an appreciably greater effect on the dark current level than on the signal level.

However, when a vidicon is operated in an ambient characterized by appreciable temperature fluctuation, the dark current level also uctuates. Such fluctuations occur as a consequence of changes in conductivity of the photoconductive layer in response to temperature changes.

Thus, the conductivity increases with increase in temperature and decreases with decreasing temperature. Therefore, the fluctuations in the dark current level occur upwardly in response to increasing temperature and downwardly in response to lower temperatures. The upward uctuations of the dark current level are particularly troublesome. Thus, as such upward fluctuations encroach upon the signal level it may become impossible to distinguish a signal from a nonsignal area on the target. In this way, a significant portion of the Signal may be lost. In addition, where reticles are provided on the target for the purpose of defining a par- 3,519,866 Patented July 7, 1970 rice SUMMARY OF THE INVENTION I have discovered that when gold applied to a thickness for opaqueness is employed at the dark signal reference area and the reticle areas of the target, in a novel combination with the signal electrode coating, appreciable improvement in etfectivenes of the opaque reference areas is secured. In the combination referred to, the gold layers are deposited between adjacent lms of the signal electrode. This provides a signal electrode substrate for the gold layers which facilitates application of the gold layers because of the readiness with which gold wets the signal electrode. The combination also provides a covering film of signal electrode material over the gold coated areas for the purpose of isolating the gold layers from a photoconductor formed on the signal electrode. In the absence of such protection of the gold layers from the photoconductive layer, there is danger that the photoconductive layer may react adversely with the gold layers and reduce their effectiveness in providing opaque areas.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary and partly sectional view of a photoconductive pickup tube in accordance with my disclosure, and

FIG. 2 is a sectional view taken along the line 2--2 of FIG. l and shows more clearly the dark current reference area as well as the reticle areas.

DESCRIPTION OF THE PREFERRED EMBODIMENT the other end portion of the envelope 12 is positioned` an electron gun (not shown) for providing an electron beam. The electron beam is scanned across the target 20 by suitable means such as electromagnetic coils (not shown) disposed outside of the tube envelope 12.

The target 20 is a multilayer structure. It includes a relatively thin first film 26` of tin oxide having a thickness to provide an electrical resistance of from about 50,000 ohms to l megohm per linear inch. 'This thickness is estimated to be about angs'troms. Before application of the rst film 26 of tin oxide to a surface of the faceplate 14, the latter is cleaned in a suitable solution, rinsed and hot air dried. The faceplate 14 is then placed in an oven and tin oxide is applied to the aforementioned surface of the faceplate by atomizing a solution of stannous chloride with nitrogen. The temperature of the oven is about 450 C. The atomized solution is carried to the faceplate 14 by suitable means such as a glass chimney (not shown) while the faceplate is in the heated oven. When the stannous chloride solution is carried by the chimney to the oven, there is formed within the oven tin oxide by pyrolytic decomposition which deposits on the faceplate 14 and uniformly forms the first film. 26l of tin 3 oxide thereon. This coating procedure as well as the several coating applications to be described are carried out prior to sealing the faceplate 14 to the bulb 12.

After application of the first tin oxide film 26, gold, having purity of 99.98% is evaporated over the first tin oxide film 26 through a nickel mask having a pattern for forming a dark current reference area 28 and reticle areas 30, 32, 34, 36 as shown in FIG. 2. The gold evaporation is effected in a bell jar having a pressure less than 1 10-6 mm. of mercury. A 1boat (not shown) in which the gold is carried during the evaporating step, is heated to a temperature of about 1100 C. The boat may be made of alumina coated tungsten. To assure smoothness of the gold layers formed in this operation, relatively small pellets of alumina are included with the gold in the evaporating boat. The pellets may have a diameter of about 15 mils. The presence of the alumina pellets in the boat prevents superheating and sputtering of the gold, thus assuring an absence of gold particles in the layers formed. The gold layers so formed adhere well to the first tin oxide film 26 over which they are applied, as a consequence of a wetting of the tin oxide film by the gold. The gold layers formed in accordance with the foregoing have a thickness sufficient to provide opaqueness to light. This thickness is at least about 400l angstroms and is determined by a suita'ble light monitor.

After the gold layers have been formed to provide the dark current reference area 218 and the reticle areas 30, 32, 34, 36 (FIG. 2) the faceplate 14 is removed from the bell jar and returned to the tin oxide coating oven Where a second -film 40 (FIG. 1) of tin oxide is formed over the first film 26 of tin oxide and over the opaque reference layers of gold, referred to. The second tin oxide film 40 is formed in a manner similar to the formation of the first tin oxide film 26 as described in the foregoing. The second film 40 of tin oxide is deposited to a thickness appreciably greater than that of the first tin oxide film 26 so that the combination of the first and second lms of tin oxide provide a lateral resistance of about 60100 ohms per lineal inch. The combined thickness of the two tin oxide layers 26 and '40 is estimated to be about 2200 angstroms.

After application of the second tin oxide lm 40, a layer 42 of photoconductive material is applied thereover. The layer 42 of photoconductive material may be a solid layer of a mixture of 67% antimony trisulphide and 33% antimony oxysulphide, by weight. The thickness of the photoconductive layer `42 may be aboiut from 3 to 4 microns. The layer 42 of photoconductive ma- 4 terial may be applied in a manner described in U.S. Pat. 3,127,226 issued to C. W. Rector on Mar. 31, 1964.

The disposition of the gold layers forming the reference areas 28, 30, 32, 34 and 36 between two layers of tin oxide in accordance with my disclosure, is accompanied by several advantages. It promotes adherence of the gold layers as a consequence of a wetting of the first tin oxide film 26 `by the gold. The second film 40 of tin oxide is deposited not only over the first film 26 but also over the gold layers constituting the reference areas. Such covering of the gold layers isolates the gold from the photoconductive layer 4-2, thus precluding any adverse reaction of the gold with the photoconductive layer. In addition; the total coverage of the gold reference areas and the lfirst tin oxide film 26 facilitates a removal of the photoconductive layer 4-2 if it should prove unacceptable, withoiut harming the gold layers and the tin oxide lms. The removal of the Iphotoconductive layer V42 can be accomplished by washing the faceplate in a caustic solution or a suitable acid. Such washing leaves the faceplate in condition for receiving a new photoconductive layer.

-I claim:

1. A target for a photoconductive pickup tube comprising:

(a) an insulating substrate,

(b) a layer of photoconductive material over said substrate, said photoconductive material being a mixture of antimony trisulphide and antimony oxysulphide, and

(c) a composite layer intermediate said substrate and said layer of photoconductive material, said composite layer comprising a tin oxide layer having an opaque pattern of gold deposited therein and completely encapsulated there'by.

2. A target according to claim 1 and wherein said composite layer has a resistivity of about 6000 ohms per lineal inch.

References Cited UNITED STATES PATENTS 3,290,530 12/196'6 Heagy.

3,310,700 3/1967 Dresner et al.

3,313,971 4/1967 Nagy 313-94 X 3,346,755 10/1967 Dresner 313-94 3,418,508 12/1968 Bynum 313-1095 X ROBERT SEGAL, Primary Examiner U.S. C1. X.R. 

